Week 8Galaxies

Reading:

Chapter 15, Sections 1, 3 (9 pages)

Galaxy/Not a Galaxy?

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Types of Galaxies

The Hubble Classification System:• Spirals• Barred Spirals• Ellipticals• Irregulars

Spiral GalaxiesHave nuclear bulges and spiral arms.

Spiral arms have clouds that are forming new stars.

Three Sub-types:

Nuclear Bulge

Spiral Arms

Sa Big Tightly wound

Sb Moderate Moderately wound

Sc Small Loosely wound

Spiral galaxies are classified according to the size of their central bulge and the tightness of their arms.

Spiral Galaxies (Face-on)

Sa Sb Sc

Tight arms, big bulge

Loose arms, small bulge

Spiral Galaxies (Edge-on)

Sa Sb Sc

Tight arms, big bulge

Loose arms, small bulge

Face-on

Edge-on

Barred Spiral Galaxies• Like normal spirals, but have central bars

• Bars are extended, linear bulges

• Half as common as “normal” spirals

SBa SBb SBc

Spiral Density Waves

Spirals arms like traffic jams – jam always there, but different cars.

Stars and gas clouds rotate around galaxy faster than the spiral density wave.

Clouds get compressed when passing through wave; new star formation is triggered.

Spiral Density Waves

Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of less than a million stars.

Ellipticals also contain very little, if any, cool gas and dust, and show no evidence of ongoing star formation.

Elliptical Galaxies

Elliptical Galaxies

Elliptical in shape

Mostly older stars (note yellowish color)

Seven Sub-types: E0, E1, E2, E3, E4, E5, E6, and E7

E0 – almost perfectly round

E3 – somewhat elongated

E7 – very elongated

Elliptical Galaxies

E0 E3 E6

Ellipticals are classified according to their shape from E0 (almost spherical) to E7 (the most elongated). Which class depends on angle; E0 can look like E7 if seen “edge-on”.

Giant Elliptical Galaxies

~20x as big as the Milky Way

Dwarf Elliptical Galaxies

So few stars that you can see right through them.

S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas:

Irregular galaxies have a wide variety of shapes. Both these galaxies appear to be undergoing interactions with other galaxies.

Irregular Galaxies

Irregular Galaxies

LMC = Large Magellenic Cloud

SMC

www.galaxyzoo.org

Galaxy classification tutorial

Galaxy Zoo

How We Know that Galaxies are Far Away

Cepheid variables

1912 – Henrietta Leavitt’s Cepheid Period-Luminosity Relation

Cepheid Variables in Andromeda

1923 – Edwin Hubble discovers Cepheids in the Andromeda “Nebula”, M31. It is 2.2 million light-years beyond the Milky Way

The Local Group

The Local Group is the cluster of galaxies to which the Milky Way belongs.

• LG is relatively poor: ~40 galaxies

• Andromeda (M31): largest and most massive

• More than 1/3 are dwarf ellipticals

• 3 are spirals (MW, M31, M33)

• New dwarf ellipticals being discovered

Here is the distribution of galaxies within about 1 Mpc of the Milky Way.

The Local Group

A nearby galaxy cluster is the Virgo cluster; it is much larger than the Local Group, containing about 3500 galaxies.

Most galaxies are too far away to pick out their Cepheid variables.

How do we measure distances to them?

Using atomic spectra, Doppler shifts and the Hubble law.

Very Distant Galaxies

Cosmic Fingerprints

Spectral lines are like fingerprints – they identify the element that produces them.

We use these fingerprints to study the chemical composition and distances of objects in space.

Doppler Shifts

A moving source of light appears to produce different lines (fingerprints) than a stationary source of light.

This effect is called a Doppler Shift.

That is, a moving object’s fingerprints will be shifted with respect to the fingerprints from a stationary object.

The Doppler Effect

Depends only on the relative motion of source and observer.

If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer.

Light moving towards you is blueshifted.

Light moving away from you is redshifted.

Most galaxies are too far away to pick out their Cepheid variables. Their distances are instead found through their Doppler shifts.

All galaxies seem to be moving away from us, with the redshift of their motion correlated with their distance:

Very Distant Galaxies

These plots show the relation between distance and recessional velocity

Hubble’s Law

The relationship (slope of the line) is characterized by Hubble’s constant H0:

The value of Hubble’s constant is currently uncertain, with most estimates ranging from 50 to 80 km/s/Mpc.

Note that the Universe is expanding. More on this in the last week of class.